Electroplated articles having molybdenum base metal



March 15, 1960 m. BEACH ET AL J ELECTROPLATED ARTICLES HAVING MOLYBDENUM BASE METAL Original Filed Jan. 7, 1957 .003 -.005 INCH CHROM/UM GLENN 1?. SCHAEE IUZZHVE'YS United States Patent ELECTROPLATED ARTICLES HAVING MOLYB- DENUM BASE METAL John G. Beach and Glenn R. Schaer, Columbus, Ohio, assignors, by mesne assignments, to the United States gr America as represented by the Secretary'of the Original application January 7, 1957, Serial No. 632,951,

now Patent No. 2,886,499, dated May 12, 1959. Divided and this application October 30, 1958, Serial No. 770,903

6 Claims. (Cl. 29-194) This application is a division of a copending application entitled, Protective Metal Coatings for Molybdenurn, Serial No. 632,951, filed January 7, 1957, now Patent No. 2,886,499, by the inventors, Glenn R. Schaer and John G. Beach.

The present invention rel-ates to electroplated articles and more particularly to electroplated articles that have molybdenum for a base metal.

It is well known. that molybdenum, as well as other refractory metals, is readily oxidized when exposed to an oxygen-bearing atmosphere at elevated temperatures. As

molybdenum and molybdenum base alloys have high strengths at elevated temperatures, it is desirable to provide satisfactory oxidation resistance for these materials so that their use can be expanded. It has been the customary practice to electroplate molybdenum with various other metals, such as nickel and chromium, to prevent the P ce Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

Figure 1 is a partial sectional view, showing chromium and gold plating on a molybdenum specimen; and

Figure 2 is a sectional view illustrating a microscopic view of an electroplated molybdenum specimen, after heating.

As shown in Figure 1 of the drawing, the article of the present invention is made by first applying a strike coating (approximately 0.0005 inch) of chromium 11 on the molybdenum 12 in order that the subsequent gold plate 13 will be deposited adherently. The base metal is cleansed and prepared for plating in any conventional manner and then plated with chromium. By way'of example, the molybdenum objects to be plated can be heat treated in hydrogen for 2 hours at 980 C., cooled, im mersed in nitric acid solution (3HNO :1H O) for 10 seconds, rinsed, electro-etched 2 minutes at 350 amp/sq. ft.

oxidation of the molybdenum. A protective electroplated metal at a high temperature must not only resist oxidation from theenvironment in whichthe coating will be placed but must also prevent deterioration of the basis metal caused by the environment. The oxidation resistance of most commercially usable electrodeposited'metals is dependent upon the formation of an adherent and impervious oxide coating. In addition to oxidation resistance, the electroplate and its scale must withstand impact and abrasion, stress corrosion and creep. If the electroplate diffuses rapidly with the basis metal, the coatinglife is shortened. Discontinuities in the coating reduce its ability to protect the basis metal, and therefore the density and structure of electrodeposits are important.

Although nickel resists oxidation in air better than chromium under conditions of cyclic heating, chromium can be deposited on molybdenum with a better adherence than is possible by plating nickel and several other electrodeposits directly on molybdenum. However, at high temperatures, the molybdenum diffuses through the chromium plating, causing shrinkage of the molybdenum, and, furthermore, molybdenum and/or oxygen diffusing through the plating and reacting with each other at high temperatures form a liquid or gas that corrodes chromium, nickel, and other metals faster than does air.

The present invention comprises an improved molybdenum or molybdenum base alloys that have a chromium layer plated thereon and a deposit of gold that has been applied over the layer of gold. The article can also have a protective layer of chromium deposited over the layer of gold. At elevated temperatures, gold only slightly retards the diffusion of chromium through the gold and into the molybdenum, but diffusion of the molybdenum through the resulting gold-chromium diffusion alloy is effectively retarded.

It is therefore a general object of the present invention to provide an improved molybdenum base article whereby the rapid oxidation of the molybdenum at high temperatures is efiectively retarded.

( F.) in a solution containing g./l. chromic acid and 10 g./l. sodium dichromate, rinsed, immersed in 15 percent, by weight, of hydrochloric acid solution, rinsed,

and plated with chromium at F. with a current density of 900 amp./ sq. ft. for 1 to 5 minutes.

, After the molybdenum specimens have been given a strike coating of chromium, a .0015 inch gold layer is deposited. By way of example, the specimens are immersed f in 15 percent, byweight, hydrochloric acid solution for 5 seconds at 85 F., rinsed, and then plated in a gold plating bath, having the following formulation and operating conditions:

FORMULATION 40 g./l. potassium gold cyanide 29 g./l. potassium cyanide 40 g./l. potassiumvtartrate 3 g./l. potassium hydroxide 10 g./l. potassium carbonate 12.0 pH

Operating conditionsl0 amp/sq. ft.

After plating with gold, a heavier coating of chromium 14 (.003-.005 inch) is then deposited over the layer of gold. By way of example, the heavier layer of chromium 14 can be plated by using the following formulation and operating conditions:

' FORMULATION 250 g./ l. chromic acid 2.5 g./l. sulfuric acid Operating conditions Cathode current density 750 amp/sq. ft.

Anodes Chemical lead sheet.

Bath temperature -2--- 185 F.

Plating time Approximately 3-5 hours.

Referring now to Figure 2 of the drawing, there is shown a view, as might be, observed under a microscope,

of an electroplated molybdenum specimen that has been heated, wherein-the heating caused a progressive growth of a molybdenum-chromium diffusion zone 15 under the gold. Microscopic thickness measurements of sections heated for 25, 57, and 100 hours showed that the'alloy zone grows atnearly the same rate as the diffusion alloy 3 grows between molybdenum and chromium. For example, one molybdenum specimen that was first given a .0005 inch thick chromium strike, followed by a .0015 inch .platingpf gold, andthen -.004 inch ofchromium, was heatedfor. 100 hours at, 080"; C. inargon atmosphere, and the .difiusionlonein the gold and chromiumplated molybdenllm..-section,.sho.wn in Figure 2,.was about .00075 [inch thick,. whereas the difiusion zone.,.after.. l00,hours .for,.

chromium plated, .molybdemlm:..(with0llt gold) 'wouldbe about .0009inchthick...

The..difius ion...zone. 15, consistingof two layer-like phases betweenthe gold andv the molybdenum, is a white alloy with a hardness of 470 Knoop. It contained little, if, any, gold, because the intermetallic compounds of gold and molybdenummrevery much .harder. The hardness of the .yellow-gold layer. 16 above the chromium-molybdenum zone .15indicates that, uponheating, a significant proportion of chromium isalloyed .with. the gold... The hardness ofthegold-chromium alloy 16 is 199 Knoop, whereas annealed .gold has a hardness of less than 40 Knoop. The hardness of the chromium 14 above the gold alloy is. 180 Knoop, and, because this value corresponds to the-hardness of annealed, electrolytic "chromium, it is evident that. no molybdenum or gold diffuses into the outer layenof chromium... Thus it is evident that 'the molybdenum ,base metal 12, is adequately protected from oxidationat hightemperaturesrby thelayersof chromium and gold.

During microscopic examination ,of -electroplated 'molybdenum specimens, several gaps in the gold alloy layer were observed... Whenever the gapsappeared, the white difiusion alloy vbelow the. gold alloy was very narrow. These observations show that the gap vretarded the diffusion of chromium with molybdenum. Because the growthof the white diffusion zone depended upon the difiusion of chromium through gold, the white difiusion zone evidently contained little, if any, gold.

After molybdenum or molybdenum base alloys are coated with a diffusion barrier as described above, it may be desirable to apply an additional oxidation resistant metal over the outer coating of chromium 14. In particular, under conditions of cyclic heating,- the electroplating of a nickel-coating (0005-001) over the chromium coating 14 is highly desirable. Although chromium is ordinarily considered more resistant than nickel to oxidation in air, nickel is actually more resistant under conditions of cyclic heating, the reason for this apparently being due to the fact that the nickel-forms a tightly adherent nickel oxide surface coating. On the other hand, the chromium oxide scale formed on the chromium surface is not as adherent as the: nickel oxide scale formed on the nickel surface.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings: it. shouldtherefore' be understood thatzit is intended that all-matter-containedin-the=-above description. and drawing- ;shalhbe interpreted ;.as. illustrative and notina limiting sense; I

What is claimed: is; 1. An article composed of molybdenum, a strikej coatingot chromium platedthereon, a layer, ofgold plated...

on said strike coating of chromium, and a substantially thicker layer of chromium plated on said gold.

2. An article composed of molybdenum, a strike coating of chromium about .0005 inch thereon, a layer of gold about .0015 inch pl-ated-onsaid strike 'coating of chromium, and a layer of chromium from .003 to. .005 inch thickplated on said gold; 7

3. An article coinposed'of molybdenum, a'strike coat ingot chromium plated thereon, alayerxof gold plated on-said' strike coatingof chromium, a layer of chromium from .003 to .005 inch thick plated on said layer of gold; and an outside coating of nickel plated on said layer of chromium.

4. .An article composed of,molyb'denum, astrikecoat ing of chromiumabout .0005 inch thereon, a layer ofgold.

about .0015 inchplated on said strike coating fof chromium, a'iayer ofj'chromium from ,003 'to2005 inch thick plated on said gold, and an outside coating of nickelv from .0005 to .001 inch thick plated onsaid layer of chromium.

5'. Anarticie composed of molybdenum base metal, a layer of"molybdenum-chromiumalloy adjacent said base metal, a layer of gold-chromium alloy adjacent said layer of molybdenum-chromium alloy; and a layer of chromium adjacent said gold-chromium alloy.

6. An article composed of molybdenum base metal as set'forth in claim 5 wherein an outside coating of nickel is plated on said layer of chromium.

References Cited in the file-of-thispatent UNITED .STATES PATENTS 1,650,979" Brace Nov. 29,1927 2,401,040; Becker May 23, 1946 2,492,204 Van Gilder Dec. 27; 1949 2,697,130 Korbelak' Dec. 14, 1954 

1. AN ARTICLE COMPOSED OF MOLYBDENUM, A STRIKE COATING OF CHRONIUM PLATED THEREON, A LAYER OF GOLD PLATED ON SAID STRIKE COATING OF CHROMIUM, AND A SUBSTANTIALLY THICKER LAYER OF CHROMIUM PLATED ON SAID GOLD. 